Oilfield operations may be performed to locate and gather downhole fluids, such as those containing hydrocarbons. Wellbores may be drilled along a desired trajectory to reach one or more subterranean rock formations containing the hydrocarbons and other downhole fluids. The trajectory may be defined to facilitate passage through the subterranean rock formation(s) and to facilitate production. The selected trajectory may have vertical, angled, and/or horizontal portions. The trajectory may be selected based on vertical and/or horizontal stresses of the formation, boundaries of the formation, and/or other characteristics of the formation.
Natural fracture networks extending through the formation also provide pathways for the flow of fluid. For example, fracturing operations may include creating and/or expanding fractures in the formation to create and/or increase flow paths within the formation, such as by injecting treatment fluids into the formation via a wellbore penetrating the formation. Fracturing may be affected by various factors relating to the wellbore, such as the presence of casing and cement in a wellbore, open-hole completions, and the intended spacing for fracturing and/or injection, among other examples.
Fracturing operations may also include perforating operations, such as may be performed in a cased wellbore to make it possible for reservoir fluids to flow past the casing into the cased wellbore. Perforations may be formed using various techniques to cut through casing, cement, and/or the formation.
In vertical wellbores, and under a wide range of rock stress states such as may include the abnormal stress conditions referred to as strike-slip faults, hydraulic fractures may initiate and propagate in a vertical plane that extends longitudinally relative to the wellbore (i.e., along a plane that contains the wellbore axis). Vertical hydraulic fractures created in hydrocarbon-bearing sedimentary formations may have substantially higher hydrocarbon productivity than horizontal fractures. This is due to the anisotropic permeability of sedimentary formations, such as where the horizontal permeability is substantially greater than the vertical permeability.
The environment for creating fractures is more complex in horizontal wellbores. In a normal stress state (i.e., where vertical stress components due to overburden pressure is substantially greater than horizontal stress components), the hydraulic fractures created from a horizontal wellbore via perforations extending normal to the wellbore initiate in a vertical plane that extends longitudinally relative to the wellbore. As hydraulic fracturing fluid is pumped into the wellbore, the fractures propagate along the same plane, but at distances further away from the wellbore, the direction of the fractures changes to follow a vertical plane that is parallel to the direction of the maximum horizontal stress components. Such change of direction results in complex fluid pathways extending from the hydraulic fractures to the wellbore, resulting in a bottleneck that reduces the overall hydraulic conductivity of the fractures, and adversely impacting hydrocarbon productivity.
In a horizontal wellbore extending into a formation under abnormal stress state, such as a strike-slip fault state where vertical stresses are between the maximum and minimum horizontal stress components, the fractures also initiate in a plane extending longitudinally relative to the wellbore if normal perforations are used. However, under strike-slip conditions, the initiation plane is horizontal and the fractures may continue to develop in a horizontal direction. This can be detrimental to hydrocarbon production because horizontal fractures produce substantially less than vertical fractures of the same surface area in the same formation.